I don't know if Vance was aware of it, but the Cephaloscope was an actual device invented in 1842 for use by physicians who worked with hearing problems. "Cephalo" refers to the head.

Farr knew there was a cephaloscope focused on his brain. Any pulse of excitement, any flush of fear would be recorded on a chart. He brought the image of a hot bath to the brink of his mind.

“Do you plan to steal houses, Farr Sainh?”

Now: the placid cool porcelain, the feel of warm air and water, the scent of soap.

“No...”

After the samples were collected, Kirdy stepped into the saloon and made a statement. “I will question you separately. Those who so desire will be allowed to give their evidence with the cephaloscope as an adjunct, and these responses will naturally take on more weight. I remind you that cephaloscope evidence can not be introduced in court to prove guilt—only to prove innocence. The cephaloscope at worst can only fail to eliminate you from the suspects. I remind you further that refusal to use the cephaloscope is not only a privilege and a right, but considered by many a moral duty. Hence those who prefer to give evidence without cephaloscope verification incur no prejudice. Use of the instrument is optional with you.”

The modern lie detector was invented by Dr. William M. Marston in the 1917. The machine was also called a polygraph - literally "many writings", referring to the method of recording several physiological responses at the same time. He also wrote under the pen name Charles Moulton - creating the Wonder Woman comic strip. Wonder Woman, as you may recall, had a magic lasso that caused anyone she caught with it to tell the absolute truth.

Compare to the truth meter From The Star Beast (1954) by Robert Heinlein, the Veridicator, from the 1962 novel Little Fuzzy by H. Beam Piper.